Semiconductor light-emitting devices such as Light-Emitting Diodes (LEDs) and laser diodes typically utilize an active layer, where electrons and holes combine to emit light. Table 1, which is presented below, describes the epitaxial layer structure of an LED that emits light at a wavelength of 345 nm. The structure utilizes multiple quantum well active layers with homogeneous composition. In Table 1, the layers are numbered in order from 1 to 426, with 1 being the top most layer and 426 the layer closest to the substrate. In order to save space, in situations where a specific sequence of layers is merely repeated a number of times, the number of times that the original sequence repeats is indicated in the “repetition” column of Table 1. For example, although layers 4-43 are not specified, the sequence of layers 2 and 3 is repeated 20 times. Thus, all even layers between 2 and 44 (layers 4, 6, 8, 10, . . . , 38, 40, 42) are the same as layer 2, while all odd layers between 3 and 45 (5, 7, 9, . . . , 39, 41, 43) are the same as layer 3.
The “composition” column of Table 1 is self-explanatory, as is the “thickness” column. Some layers are indicated as having a dopant, for example, layer 2 (Al0.30Ga0.70N:Si) is doped with silicon. The “comments” column of Table 1 provides additional descriptive information about the layer or group of layers. For example, layers 49 and 53 are the quantum well layers and have a relatively low band gap, while layers 46-48, 50-52, and 54-56 serve as the barriers and have a relatively high band gap compared to the quantum well layers.
TABLE 1ThicknessLayerComposition(nm)RepetitionComments1GaN: Mg20p-contact2Al.30Ga.70N: Si3.0620p-cladding,144.48 nm3Al.28Ga.72N: Si3.82total44Al.26Ga.74N: Mg66.96p-waveguide45Al.48Ga.52N: Mg21.88tunnel barrierlayer46InxAl(.18−x)Ga.82N4.06barrier47InxAl(.18−x)Ga.82N: Si2.3248InxAl(.18−x)Ga.82N4.0649InxAl(.15−x)Ga.85N5.25quantum well50InxAl(.18−x)Ga.82N4.06barrier51InxAl(.18−x)Ga.82N: Si2.3252InxAl(.18−x)Ga.82N4.0653InxAl(.15−x)Ga.85N5.25quantum well54InxAl(.18−x)Ga.82N4.06barrier55InxAl(.18−x)Ga.82N: Si2.3256InxAl(.18−x)Ga.82N4.0657InxAl(.26−x)Ga.74N: Si70.56n-waveguide58Al.28Ga.72N: Si3.16100n-cladding,555.22 nm59Al.30Ga.70N: Si2.33total260Al.30Ga.70N: Si0.728261Al.31Ga.69N: Si1198.35n-contact layer262Al.31Ga.69N159.78interface layer tolow Al263GaN0.2540strain relief layer264AlN0.38345GaN0.2539346AlN1.0425AlN28.8surfaceconditioning layer426Al.83Ga.27N1200AlGaN templateSapphire substrate
The active region of the 345 nm LED described above in Table 1 are layers 46-56. These layers include three barrier layers and two 5.25 nm-thick quantum wells composed of InxAl(0.15-x)Ga0.85N with x—the indium component—nominally at 1%.
Table 2, which is presented below, summarizes the Metal Organic Chemical Vapor Deposition (MOCVD) gas flow conditions for each of the quantum wells (layers 49 and 53) of Table 1. A constant flow rate of 0.9 cc/min of tri-methyl gallium (TMG), 0.4 cc/min of tri-methyl aluminum (TMA), and 80 cc/min of tri-methyl indium (TMI) is employed during the 120 second growth time of each quantum well.
TABLE 2Time(sec)TMG (cc/min)TMA (cc/min)TMI (cc/min)0-1200.90.480
Most light-emitting semiconductor devices such as LEDs and laser diodes employ quantum wells with uniform material compositions, such as the quantum wells described above in Table 1. Thus, conventional quantum well designs employ very uniform material and require the crystal grower to go to great lengths to ensure that the quantum well material is as homogeneous as possible.
While such active layers are well suited for many applications, they may not be optimal for some material systems, including those necessary for accessing the green and deep UltraViolet (UV) wavelength regions. The green wavelength region may be considered to be from about 470 nanometers (nm) to about 550 nm, while the deep UV wavelength region may be considered to be from about 200 nm to about 365 nm. For example, with InxGa1-xN, the high In component needed to attain a bandgap for green emission usually leads to uncontrollable segregation of InN or GaN during material growth. For the deep UV wavelength range, it becomes increasingly difficult to grow high quality quantum wells with the high Al-containing AlGaN. Example embodiments address these and other disadvantages of the conventional art.
FIG. 1 is a graph 100 illustrating the L-I (light vs. current) characteristics of 300 μm×300 μm deep UV LEDs that employs conventional quantum well active layers having uniform material composition. FIG. 2 is a graph 200 illustrating the L-I characteristics of 200 μm×200 μm deep UV LEDs that employ conventional quantum well active layers having uniform material composition. Graphs 100 and 200 are referenced in the description of the example embodiments below for comparison purposes.